A molten steel having been refined with a converter or a vacuum treatment vessel contains a large amount of dissolved oxygen. The excessive oxygen is generally deoxidized with Al that is a strong deoxidizing element having strong affinity for oxygen. However, Al forms Al2O3 inclusions through deoxidation, and the inclusions aggregate to form coarse alumina clusters each having a size of at least several hundreds of micrometers. The alumina clusters cause generation of surface defects during the production of a steel sheet, and greatly deteriorate the quality of the steel sheet. In particular, for a low carbon molten steel as a material for a steel sheet that has a low carbon concentration and, after refining, a high dissolved oxygen concentration, the amount of alumina clusters is very large. The generation ratio of surface defects of a steel sheet produced therefrom is extremely high, and measures for decreasing the Al2O3 inclusions remain as an important problem to be solved.
In order to decrease the Al2O3 inclusions, the following methods have heretofore been proposed and carried out: a method described in Japanese Unexamined Patent Publication (Kokai) No. 5-104219 comprising adding flux for adsorbing inclusions on a molten steel surface to remove Al2O3 inclusions; and a method described in Japanese Unexamined Patent Publication (Kokai) No. 63-149057 comprising adding CaO flux to a molten steel with the flow of pouring the molten steel utilized whereby Al2O3 inclusions are adsorbed and removed. On the other hand, Japanese Unexamined Patent Publication (Kokai) No. 5-302112 discloses, not as a method of removing Al2O3 inclusions but as a method of not forming Al2O3, a method of preparing a molten steel for a steel sheet, which comprises deoxidizing a molten steel with Mg and in which the molten steel is substantially not deoxidized with Al.
However, it is very difficult to remove Al2O3 inclusions formed in a low carbon molten steel in a large amount and to such a degree that surface defects are not formed, by the methods of removing Al2O3 inclusions explained above. Moreover, when a molten steel is deoxidized by Mg deoxidation that forms no Al2O3 inclusions, the Mg vapor pressure is high, and the yield of Mg in the molten steel is very low. Accordingly, a large amount of Mg is required to deoxidize, with Mg alone, a molten steel having a high dissolved oxygen concentration such as a low carbon steel. Therefore, it cannot be said that the process is practical when the production cost is taken into consideration.
In view of these problems, an object of the present invention is to provide a low carbon steel sheet and a low carbon steel slab on which formation of surface defects can be surely prevented by preventing aggregation of inclusions in the molten steel and finely dispersing inclusions in the steel sheet or slab, and a process for producing the steel sheet and slab.